1. Field of the Invention
This invention relates to an information recording medium, and more particulary to a phase-change-type information recording medium applicable to optical memory-related devices, which comprises a specific phase-changeable compound in its recording layer.
2. Discussion of Background
A phase-change-type information recording medium is now very popular in the field of optical information recording. The recording medium of this type utilizes a change between a crystalline phase and an amorphous phase, or between two crystalline phases of a material of the recording layer. Such a change between two different phases occurs when electromagnetic waves, in particular laser beams, are applied to the recording medium. Information recording, reproduction and overwriting can thus be achieved.
On the phase-change-type information recording medium, information can be overwritten by applying a single beam. Moreover, the recording medium of this type does not require a complicated optical system on the driving side. For these reasons, research and development of such a recording medium is extensively being made these days.
Thin films of Ge-Te, Ge-Te-S, Ge-Se-S, Ge-Se-Sb, Ge-As-Se, In-Te, Se-Te and Se-As, disclosed in U.S. Pat. No. 3,530,441, are known as typical chalcogen alloy thin films usable as the recording layer of the phase-change-type information recording medium.
In addition, aiming at improving the stability and achieving high-speed crystallization, the following proposals have been made so far. Namely, a Ge-Te alloy thin film containing Au, as disclosed in Japanese Laid-Open Patent Application 61-219692, a Ge-Te alloy thin film containing both Sn and Au, as disclosed in Japanese Laid-Open Patent Application 61-270190, and a Ge-Te alloy thin film containing Pb, as disclosed in Japanese Laid-Open Patent Application 62-19490.
In order to obtain an improved recording medium which can withstand repeated information recording and erasing, a material composed of Ge, Te, Se and Sb with a specific composition ratio has also been proposed in Japanese Laid-Open Patent Application 62-73438.
None of the above recording media, however, has all the characteristics required for a phase-change-type rewritable optical information recording medium. The remaining problems to be solved are improvement of the recording and erasing sensitivities, prevention of lowering of the erasion ratio, which is caused due to incomplete erasion at the time of overwriting, and prolongation of the expected life span of a non-recorded area.